Fruit flies use sunlight to stay on course

How does an insect keep its bearings while migrating for thousands of
miles? "If you go out in a field, lie on your back and look up at
the sky, that's pretty much what an insect sees," says Michael
Dickinson, a biology professor at University of Washington.

Tiny animals like monarch butterflies and locusts maintain a
constant heading while migrating across entire continents, while
bees and ants always find their way hundreds of metres back to the
nest without a problem.

Realising that insect must have some type of internal compass,
Dickinson and Peter Weir, a doctoral student at the California
Institute of Technology, decided to test how insects orient themselves by tethering fruit flies to
metal pins, and suspending them in a magnetic field.

This allows the flies to move and rotate naturally while being
held in place. The fruit flies (Drosophila melanogaster)
were taken to an arena atop a building that's taller than visual
landmarks like treetops. Digital cameras tracked the insect's
flight headings.

During the hour before and the hour after sunset, the headings
of these flies -- relative to the position of the arena -- were
recorded for 12 minutes. The entire arena was rotated 90 degrees
every three minutes, to see what the flies did.

Under natural light, some of the flies compensated for the
rotations and maintained a consistent heading. But when the arena
was covered with a circular polarising filter -- which eliminates
the natural light patterns of the Sun -- the flies did not shift
their heading significantly in response to arena rotations.

This indicates that the fruit fly has the ability to coordinate
its compound eye and brain functions for navigation by using the
using light polarisation patterns of natural sunlight. Other
insects with similar eyes and similar flight patterns likely have
the same capability.

Next up, the team wants to determine why the flies select a
particular heading. "A lot of our research is focusing on how the
fruit fly brain is multitasking in space and time to achieve
remarkable effects," Dickinson said. Fruit flies have just 300,000
neurons in their brain, while the average adult human has
100,000,000,000.